Solid state light emitting devices, including solid state lamps having light emitting diodes (LEDs), laser diodes (LDs), and resonant cavity LEDs (RCLEDs) are extremely useful, because they potentially offer lower fabrication costs and long term durability benefits over conventional incandescent and fluorescent lamps. Due to their long operation (burn) time and low power consumption, solid state light emitting devices frequently provide a functional cost benefit, even when their initial cost is greater than that of conventional lamps. Because large scale semiconductor manufacturing techniques may be used, many solid state lamps may be produced at extremely low cost.
In addition to applications such as indicator lights on home and consumer appliances, audio visual equipment, telecommunication devices and automotive instrument markings, LEDs have found considerable application in indoor and outdoor informational displays.
With the development of efficient LEDs that emit short wavelength (e.g., blue or ultraviolet (UV) radiation), it has become feasible to produce LEDs that generate white light through down conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emissions of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelengths to produce white light.
Phosphor conversion of a portion of the primary emission of the LED chip is attained by placing a phosphor layer in an epoxy that is used to fill the reflector cup which houses the LED chip within the LED lamp. The phosphor is in the form of a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED chip and subsequently cured.
The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary radiation emitted by the LED chip passes through the epoxy without impinging on the phosphor particles, and another portion of the primary radiation emitted by the LED chip impinges on the phosphor particles, causing the phosphor particles to emit longer wavelength radiation. The combination of the primary short wavelength radiation and the phosphor-emitted radiation produces white light.
Current state of the art phosphor-converted white LED (pc-LED) technology is inefficient in the visible spectrum. The light output for a single pc-white LED is below that of typical household incandescent lamps, which are approximately 10 percent efficient in the visible spectrum. An LED device having a comparable light output than that of a typical incandescent lamp necessitates a larger LED chip or a design having multiple LED chips. Moreover, a form of direct energy absorbing cooling must be incorporated to handle the temperature rise in the LED device itself. More particularly, the LED device becomes less efficient when heated to a temperature greater than 100° C., resulting in a declining return in the visible spectrum. The intrinsic phosphor conversion efficiency, for some phosphors, drops dramatically as the temperature increases above approximately 90° C. threshold. Other down conversion materials like quantum dots could start to decrease in efficiency from even lower temperatures, 25 degrees centigrade.